This invention relates generally to transmitters which detect the level of liquid in a tank to produce a signal proportional thereto, and more particularly to a liquid level transmitter that is compensated for changes in temperature.
Liquid level transmitters are known which operate on the force balance principle, the transmitter being directly bolted onto the tank containing the liquid. The fluid level in either an open tank or a closed tank under pressure or vacuum is detected by a differential pressure capsule assembly which senses the difference between the weight of the liquid (level) on both sides of the capsule, and converts it into a force that is transmitted by a connecting rod to the lower extremity of a force beam. One commercially-available liquid level transmitter of this type is manufactured and sold by Fischer & Porter Co. of Warminster, Pa., this instrument being described in the Fischer & Porter Instruction Bulletin for Series 13D3493 "Pneumatic Level Transmitter" (June 1969).
In the Fishcer & Porter liquid level transmitter and in similar transmitters manufactured by other companies, the differential capsule assembly includes a pair of coupled diaphragms which define a fill space containing a hydraulic fluid that provides a non-compressible back up under high static pressures. The outer diaphragm is exposed to the liquid in the tank, whereas the inner diaphragm is exposed to atmospheric or relatively low pressures. The coupled diaphragms are linked to the lower extremity of the force beam by a connecting rod passing through a support tube which joins the housing of the capsule assembly with the body of the force balance meter, the support tube defining the low pressure process chamber. Thus the force on the diaphragms in response to the difference in pressure is transferred by the rod to the beam to deflect the beam accordingly. The force applied to the beam is converted by the force balance transducer into a signal proportional thereto.
Thermal expansion of the fill in the fill space between the diaphragms of the capsule produces a force that is imposed on both diaphragms. Since the diaphragms are matched and have equal areas and the force is applied equally and in opposition to both diaphragms, the force is cancelled and the capsule response is independent of variations in fill temperature.
In existing types of liquid level transmitters, the mechanical coupling between the differential capsule assembly and the force beam gives rise to a serious source of error.
The metal connecting rod extending between the diaphragms and the force beam is disposed within the low pressure process chamber defined by the metal support tube joining the housing of the capsule assembly to the meter body. The temperature of this rod is often at a temperature which is distinctly at variance with the temperature of the support tube. The reason for this is that the support tube responds not only to the process temperature to which the rod is also exposed, but to ambient temperatures as well. When, therefore, the rod and the support tube are subjected to different temperatures, the mechanical coupling between the diaphragms and the force beam will change length due to the coefficient of expansion of the metals.
This change in length, multiplied by the system gradient, generates a force which, when compared to the full scale operating force, will determine the magnitude of temperature error. The adverse effect of temperature changes on the support tube can readily be observed, for when an operator's hand is placed on the support tube to impart body heat thereto, there is a distinct shift in the output signal.
In order to minimize the influence of temperature on the coupling in a liquid level transmitter between the level sensor and the meter body, it is known to replace the mechanical coupling with a hydraulic coupling in the form of a flexible capillary tube extending between the fill space in the assembly and a measuring diaphragm in the meter, this diaphragm being linked to the force beam. With a hydraulic coupling of this type, changes in the length of capillary relative to that of the support tube have virtually no effect on the performance of the coupling. But should the process heat up the fill between the diaphragms of the differential assembly, the resultant expansion in the fill will generate a pressure proportional to the fluid volume and inversely proportional to the diaphragm compliance, thereby simulating a change in liquid level and producing a spurious signal.